As critical dimensions (CD) are getting smaller and smaller, cleanliness requirements of high-vacuum environments become more and more demanding. Wherever a sensitive process (e.g., a manufacturing or inspection process) is performed in vacuum, there are requirements for the allowed molecular contents in the vacuum, which defines the level of cleanliness.
Some of the molecules contained in a vacuum chamber are already present in the air or are attached to the surfaces of the chamber prior to pumping. Other molecules outgas from solid and/or fluid materials inside the vacuum chamber, for example from the stage carrying the wafer and its associated components. Therefore, in cases where high levels of cleanliness are required, materials having low outgassing and low vapor pressure characteristics are used. The list of such materials includes, among others: stainless steel, glass, ceramics, copper and aluminum. Other materials (with medium or even low outgassing characteristics, so-called “compromise materials”) needed for mechanical and electrical modules inside the vacuum chamber contribute to the contamination of the vacuum. Such materials are carefully selected and undergo dedicated cleaning process. The list of compromise materials includes, among others: PTFE (polytetrafluoroethylene) elements, PTFE coated electrical cables, elastomers, adhesives, lubricants and electrical components. Nevertheless, such materials are a source for contaminants due to, e.g., outgassing.
Dynamic effects also contribute to the contamination of vacuum, for example: cables that are continuously bent by reciprocating motion tend to break mechanically after long run times. This is the last step of a continuous process of molecules being damaged and outgassed. Also, lubricants interact under the mechanical load of bearings, and some of the molecules are damaged and outgassed.
There is a continuous trend of developing sensitive processes in vacuum, such as semiconductor wafer manufacturing, inspection and process control processes. These state of the art processes, and the layers of semiconductor devices subjected to or formed by such processes, tend to be more sensitive to cleanliness than in the past. Consider, for example, a SEM (scanning electron microscope) that is used for process control (e.g., inspection, metrology and review). A specimen (e.g., a wafer) is placed in vacuum and is exposed to contaminants outgassed from the various materials from which the SEM is built. Consequently, mechanical and electrical modules of the SEM need to be adapted to the higher cleanliness requirements of today.
The trivial approach for achieving high levels of cleanliness is keeping whatever possible outside of the vacuum and improving the cleanliness of the components inside the vacuum. In order to reduce the contaminants contributed by the stage structure, according to certain approaches, the motors and sensors are placed outside of the vacuum chamber and only the slides and rails are placed inside of the vacuum chamber. This approach is relatively easy to implement for a one axis stage structure, or where low position accuracy is sufficient. Another approach utilizes a one axis stage structure with magnetic coupling: Two stage structures are built, one in the vacuum (the vacuum stage structure), without motor and cables, and another stage structure, with magnet load, is built in parallel outside of the vacuum (the atmospheric stage structure) and is magnetically coupled with the vacuum stage structure. In operation, the controlled atmospheric stage structure runs, and the vacuum stage structure tracks its motion by means of the magnetic coupling. Yet another approach makes use of a two axis stage structure having rotary motors located outside of the vacuum chamber and a mechanical gear drive passing the motion for the lower and upper axis.
Furthermore, equipment vendors use vacuum rated modules, made of allowed materials, e.g., low outgassing materials, and continuously develop, improved cleanliness components and cleanliness processes (e.g., cleaning by temperature bake out). An example of high cleanliness cable components are PTFE high cleanliness cables commercially available from Gore™. It should be noted that, as the sensitivity to cleanliness is increasing, the testing capability of outgassing characteristics of materials or modules is limited. It might therefore be difficult to measure the level of cleanliness of a module (e.g., the amount of contaminants outgassed from the module) and evaluate its compatibility with the cleanliness requirements of sensitive processes.